1. Field of the Invention
The present invention relates to regenerative direct current motor controls which provide regenerative braking by reversing the motor field. More specifically, the present invention is directed to such a control having an improved means for regulating the energization of the motor armature.
2. Description of the Prior Art
In regenerative direct current motor controls, the braking of the motor necessary for precise speed regulation or other desired operating characteristics is obtained by applying the power generated in the motor during braking back to the active power source for the motor, such as a.c. supply lines.
Regenerative braking is advantageous in that, with proper control of armature current, braking may be done on a permanent basis, whereas resistive braking or other methods, such as plugging, are normally utilizable only in transient conditions or for isolated stops. Braking may also be accomplished very rapidly by regeneration.
Power may be regenerated or returned to the active motor power source by maintaining the same polarity of motor armature voltage while reversing the direction of armature current flow, as in "armature reversing" regenerative motor controls. Motor power may also be regenerated by maintaining motor armature current flow in the same direction while reversing the polarity of the armature voltage. Armature voltage reversal is obtained by reversing the motor field. A motor control employing this technique is termed a "field reversing" regenerative control and is the type to which the present invention is directed. A field reversing control includes a field circuit for reversing the motor field and an armature circuit for coordinately controlling the motor armature current.
In regenerative braking operation, the reversal of the motor field reverses the polarity of the motor flux and the armature counter e.m.f., assuming the direction of motor rotation remains instantaneously the same. The reversed counter e.m.f. biases the thyristors in a thyristor bridge between the a.c. supply lines and the motor armature for current conduction in the same direction as during motoring anytime the counter e.m.f. is more negative than the a.c. supply lines voltage. This will include a portion of the negative half cycles of the alternating current power, thus permitting the reversal of voltage necessary for regeneration.
A problem in the design and operation of field reversing regenerative motor controls in the past has been proper regulation and protection of both the motor and control in the transient period of the field winding energization occurring at the transition from motoring to regenerative operation, and vice versa. During this time interval the motor field weakens in one direction of current flow, reverses, and increases in the other current flow direction. The transient period occurs because the reversal of the field winding current necessary for regeneration cannot be achieved instantaneously due to the inductance of the motor field winding.
In the armature circuit, it is highly desirable to have small or zero armature current under weak field conditions thereby to prevent speed increases as field strength weakens and commutator sparking due to armature reaction. U.S. Pat. No. 3,475,671, describes various ways by which deenergization of the armature circuit and a reduction of armature current may be obtained, including the technique of reversing the polarity of the controlling signal in the armature circuit in the initial portions of the transient period, prior to field reversal. This controlling signal alteration rapidly reduces and maintains the armature current at zero during this portion of the transient field.
In the circuitry disclosed in the foregoing patent, the polarity of a field winding signal is used to operate a digital logic type switch means between a pair of discrete switching states which establish the application and polarity of error signal as the controlling signal in the armature circuit.
However, once field reversal has been accomplished the gain and other control requirements of the armature circuit necessary for precise speed control tend to drive the armature current immediately to rated value in the terminal portion of the transient period after field reversal causing weak field commutator sparking and undesirable suddenness in the commencement of regenerative braking action.
The sudden reapplication of armature current may be overcome by establishing the magnitude of the armature current in accordance with the magnitude of the field winding current. Thus as the field winding current slowly increases after field reversal, in accordance with the inductive characteristics of the field winding, the magnitude of the armature current will similarly increase. U.S. Pat. No. 3,458,790 shows such a control in which the armature current is established by or "follows" the field current in the transient period.
While the armature current in the terminal portion of the transient period established by the field follower technique is less than might otherwise exist, such current may also be greater than is actually required by existing operating conditions in the motor. Excessive armature current is again present in the motoring during the terminal portion of the transient period.